Simulation of the FitzHugh-Nagumo model

The FitzHugh-Nagumo model is an example of a two-dimensional dynamical system that is simple and behaves in essence like the Hodgkin-Huxley model regarding to fast-slow phase plane. So it has two variables: one fast excitation variable $v$ and one slow recovery variable $w$. The system of equations is:

$$\begin{aligned}\frac{{dv}}{{dt}} &= f(v) - w + {I_{{\rm{stim}}}}\\ \frac{{dw}}{{dt}} &= \varepsilon \cdot (v + \beta - \gamma \cdot w)\end{aligned}$$ where $\varepsilon$ is a small parameter and $f$ is a suitable function.

Tradional parameter values are:

$$f(v) = v - {\tfrac{1}{3}}{v^3}\quad \varepsilon = 0.08\quad \beta = 0.7\quad \gamma = 0.8$$